1. Field of the Invention
The invention relates to a system wherein system components communicate via a communications bus. More particularly, the invention relates to bus interface modules for a computer systems.
2. Description of the Related Art
An exemplary communications system comprises a central processing unit and external functional modules within a computer which communicate via an internal communications bus. Another example is a communications system comprising at least two computers which communicate via an external communications bus. Each computer forms with one or more peripheral devices an individual communications system wherein communication occurs via communications buses. The peripheral devices can include, for example, external or internal disk drives, printers, scanners or data communications devices.
The computer and the peripheral devices communicate via a communications bus. The communications bus is a set of conductors connecting various functional units within a computer and connecting peripheral devices to the computer. The conductors usually extend parallel to each other in a cable, or across a substrate. The substrate is, for example, a semiconductor or a printed circuit board (PCB). The conductors are isolated from each other and are isolated from an electrically conducting substrate by a dielectric material. The thickness of the dielectric material contributes to a capacitance between a conductor and the support, and a capacitance between two adjacent conductors. These capacitances are referred to as parasitic capacitances and generally have negative influences on the transmission characteristics of the bus. The parasitic capacitances induce a propagation delay of signals travelling along the bus, and induce crosstalk between the conductors.
One example of a communications bus is a bus in accordance with a Small Computer System Interface (SCSI) standard. SCSI-II has been standardized as ANSI STD. The SCSI standard is a processor-independent standard for system-level interfacing between a computer and intelligent devices, including hard disks, floppy disks, CD-ROM, printers, scanners and many more. With the SCSI standard, up to fifteen devices can be connected to a single controller (or "host adapter") on the computer's bus. The SCSI standard allows sixteen bits to be transferred in parallel (eight-bit bus) and can operate in either asynchronous or synchronous modes. The synchronous transfer rate is up to 60 Mbit/s. SCSI connections normally use "single-ended" drivers as opposed to differential drivers. Single-ended SCSI connections can support up to six meters of cable. Differential SCSI connections can support up to 25 meters of cable. Further details of the SCSI bus are described in a book entitled "SCSI Understanding the Small Computer System Interface," PTR Prentice Hall, Englewood Cliffs, N.J. 07632, ISBN 0-13-796855-8.
Each computer and the peripheral devices comprise interface modules which, inter alia, organize communications between the computers and between a computer and its peripheral devices in accordance with a defined communications protocol.
The interface modules include a transceiver which has a receiver and a transmitter (bus driver) to allow bidirectional data transmission. For differential transmission, two lines, referred to as "noninverted" and "inverted" or as "positive" and "negative", are used to differentiate actual signal from noise. The bus driver of the transceiver includes a transmit amplifier to amplify a digital signal, and the receiver includes a receive amplifier to amplify the received digital signal.
In many applications, a digital signal is RUN-length encoded before it is fed to the bus driver so that the maximum number of consecutive logic "HIGH's" or "LOW's" are limited. However, applications using SCSI do not allow such an encoding of the digital signal. In these applications, the unconditioned data is fed to the bus. Thus, a data line on a SCSI bus may not change for an extended time duration. Under normal conditions, the combination of the parasitic capacitances and the limitation that the digital signal should not be encoded can cause saturation of the receiver. Saturation slows the receiver response so that the timing of the output response is not predictable.